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Photocontrolled Gel-to-Sol-to-Gel Phase Transitioning of meta-Substituted Azobenzene Bisurethanes through the Breaking and Reforming of Hydrogen Bonds Nagatoshi Koumura,* Masabumi Kudo, and Nobuyuki Tamaoki* Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received July 5, 2004. In Final Form: August 25, 2004 An azobenzene derivative substituted at its meta positions with two urethane moieties linked to two cholesteryl ester units forms a gel that exhibits sol-gel phase transitions upon photoirradiation as a result of trans-cis isomerization of the azobenzene unit. During the sol-gel phase transitions, hydrogen bonds, which are partly responsible for stabilizing the gels, are broken or reformed. Using circular dichroism spectroscopy, we detected asymmetrical fibrous networks in the aggregates of these gelators.
Introduction Research on low-molecular-weight organogelators has increased enormously during the past decade.1 Organogels are materials that consist of a small amount of an organogelator and an organic liquid; these components form three-dimensional fibrous networks as a result of the organogelator molecules assembling into one-dimensional fibrous structures by means of several driving forces, such as hydrogen bonding, π-π stacking, van der Waals forces, and charge-transfer interactions.2-5 Interestingly, the morphologies of organogels can be controlled reversibly or irreversibly in response to changes in external chemical, * To whom correspondence should be addressed. Phone: +8129-861-4586. Fax: +81-29-861-4673. E-mail:
[email protected] (N.K.);
[email protected] (N.T.). (1) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (c) Gronwald, O.; Snip, E.; Shinkai, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 148. (d) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (2) (a) Hanabusa, K.; Okui, K.; Karaki, K.; Kimura, M.; Shirai, H. J. Colloid Interface Sci. 1997, 195, 86. (b) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Diederik, B. A. R.; de Haas, M. P.; Klapwijk, T. M.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393. (c) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem.sEur. J. 1999, 5, 2722. (d) de Loos, M.; van Esch, J. H.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613. (e) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem.s Eur. J. 2002, 8, 2684. (f) Frkanec, L.; Jokic´, M., Makarevic´, J.; Wolsperger, K.; Zˇ inic´, M. J. Am. Chem. Soc. 2002, 124, 9716. (g) Miyawaki, K.; Harada, A.; Takagi, T.; Shibakami, M. Synlett 2003, 15, 349. (h) George, M.; Weiss, R. G. Chem. Mater. 2003, 15, 2879. (i) George, M.; Snyder, S. L.; Terech, P.; Glinka, C. J.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 10275. (j) Wang, G.; Hamilton, A. D. Chem. Commun. 2003, 310. (k) Sumiyoshi, T.; Nishimura, K.; Nakano, M.; Handa, T.; Miwa, Y.; Tomioka, K. J. Am. Chem. Soc. 2003, 125, 12137. (l) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278. (3) (a) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (b) Lu, L.; Cocker, T. M.; Bachman, R. E.; Weiss, R. G. Langmuir 2000, 16, 20. (c) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399. (d) Pozzo, J. L.; Clavier, G.; Rustemeyer, F.; Bouas-Laurent, H. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2000, 344, 101. (4) Ayabe, M.; Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 2744. (5) (a) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Ishi-i, T.; Yoshihara, K.; Shinkai, S. J. Org. Chem. 1999, 64, 2933. (b) Ishi-i, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001, 17, 5825. (c) Snip, E.; Koumoto, K.; Shinkai, S. Tetrahedron 2002, 58, 8863. (d) Kawano, S.; Fujita, N.; van Bommel, K. J. C.; Shinkai, S. Chem. Lett. 2003, 32, 12. (e) Babu, P.; Sangeetha, N. M.; Vijaykumar, P.; Maitra, U.; Rissanen, K.; Raju, A. R. Chem.sEur. J. 2003, 9, 1922. (f) Suzuki, M.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Helv. Chim. Acta 2003, 86, 2228. (g) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 1229.
photochemical, or thermal stimuli. We are interested in changing the shapes of molecules, potentially reversibly, by using light energy as an external stimulus. Azobenzene is one of the smartest molecules among all known photochromic compounds, and a large number of applications are based on the use of azobenzene skeletons. Many gelators incorporating azobenzene units have been synthesized and studied for their properties,6 but there are only a few examples6a,6g of the successful preparation of photoresponsive compounds that operate in the gel state through photoinduced trans-cis isomerization. This situation has arisen because the azobenzene units in these gels have difficulty changing their structure under photoirradiation conditions because they exist in particularly tightly packed aggregates formed by relatively strong intermolecular hydrogen bonding and π-π stacking interactions. Recently, we demonstrated that diacetylene cholesteryl esters having two urethane linkages result in colorless gels of nonpolar solvents and that these esters can be polymerized in the gel state upon irradiation with light to form orange-to-dark-blue-colored gels constructed from polydiacetylene nanowires.7 Because the chemical structure of these gelators consists of a pair of interaction sites, namely, two urethanes moieties that undergo hydrogen bonding and two cholesterol units that experience van der Waals interactions, we obtained very stable gels in which subsequent photopolymerization can occur as a consequence of the highly ordered molecular packing. On the basis of this structure and its two important interaction sites that mediate gelation, we have designed new photoresponsive low-molecular-weight organogelators that consist of meta-substituted azobenzene units as photosensitive core molecules (Chart 1). In this paper, we report the gelation properties of these new meta-substituted azobenzene gelators and their photoreversible sol-gel (6) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (b) Kobayashi, H.; Friggeri, A.; Koumoto, K.; Amaike, M.; Shinkai, S.; Reinhoudt, D. N. Org. Lett. 2002, 4, 1423. (c) Kobayashi, H.; Koumoto, K.; Jung, J. H.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2002, 1930. (d) van der Laan, S.; Feringa, B. L.; Kellogg, R. M.; van Esch, J. Langmuir 2002, 18, 7136. (e) Jung, J. H.; Shinkai, S.; Shimizu, T. Nano Lett. 2002, 2, 17. (f) Mamiya, J.; Kanie, K.; Hiyama, T.; Ikeda, T.; Kato, T. Chem. Commun. 2002, 1870. (g) Moriyama, M.; Mizoshita, N.; Yokota, T.; Kishimoto, K.; Kato, T. Adv. Mater. 2003, 15, 1335. (7) (a) Tamaoki, N.; Shimada, S.; Okada, Y.; Belaissaoui, A.; Kruk, G.; Yase, K.; Matsuda, H. Langmuir 2000, 16, 7545. (b) Nagasawa, J.; Kudo, M.; Tamaoki, N. Langmuir 2004, 20, 7907.
10.1021/la048334f CCC: $27.50 © 2004 American Chemical Society Published on Web 10/07/2004
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Chart 1. Structures of meta-Substituted Azobenzene Gelators
phase transitions that occur through the breaking and reforming of hydrogen bonds. In addition, we have detected, using circular dichroism spectroscopy, that asymmetrical aggregation of these gelators is induced by the presence of their chiral cholesterol units. Results and Discussion As depicted in Chart 1, the low-molecular-weight organogelators 1-3 each possess a central azobenzene unit that is substituted at its two meta positions with acetylene derivatives. These compounds possess the requisite features for the formation of gels, namely, two urethane moieties for strong hydrogen bonding and two cholesterol units for relatively weak van der Waals interactions. The trans isomers of 1-3 were synthesized by Sonogashira coupling of 3,3′-diiodo-trans-azobenzene with the corresponding terminal acetylene derivatives in the presence of catalytic amounts of bis(triphenylphosphione)palladium dichloride and copper iodide. Gelation was performed by first dissolving the gelators in a solvent, by heating them in a screw-capped bottle, and then cooling the system to room temperature. The gelation abilities of these compounds were studied at concentrations of 1.0 wt % in several solvents: n-hexane, cyclohexane, toluene, ethyl acetate, ethanol, diethyl ether, and carbon tetrachloride. Under these conditions, compound 1 (m ) 4, n ) 1) did not form a gel in any of these organic solvents but compound 2 (m ) 3, n ) 2) did gelatinize cyclohexane. Compound 3 (m ) 4, n ) 2) is a more powerful gelator; it gelatinized not only cyclohexane but also the relatively more polar ethanol. As displayed in Figure 1, the field emission scanning electron microscopy (FE-SEM) image obtained from the xerogel8 of 3 reveals a clear threedimensional fibrous network. The minimum concentration at which gelation was induced was 0.7 wt % (∼4.0 mM) at 25 °C. The gel-sol phase transition temperature (Tgel) of each cyclohexane gel of compounds 2 and 3 at the 1.0
Figure 1. SEM image of a xerogel of trans-3.
wt % concentration is 43 and 60 °C, respectively.9 These cyclohexane gels are stable at room temperature (∼25 °C), so the measurements of Fourier transform infrared (FTIR), ultraviolet-visible (UV-vis), and circular dichroism (CD) spectra and the irradiation experiment were performed at room temperature. To confirm the presence of hydrogen bonds in the gel state, the cyclohexane gels of 2 and 3 were investigated by FTIR spectroscopy.10 The FTIR spectra of compound 3, both in CCl4 solution and in the cyclohexane gel state, are presented in Figure 2. In the gel state, we observe signals at 3308, 1686, and 1547 cm-1 that we assign to NsH and CdO stretching vibrations and NsH bending vibrations, respectively, of the hydrogen bonded groups and a signal at 1734 cm-1 for the stretching vibration of the free CdO unit of each cholesteryl ester. On the other hand, the FTIR spectrum of 3 in a CCl4 solution displays signals at 3460 (NsH stretching), 1728 (CdO stretching), and 1508 cm-1 (NsH bending), which suggests that there is no evidence for hydrogen bonding in solution. Furthermore, the IR spectrum of compound 3 in cyclohexane at lower concentration giving a solution (0.1 wt %, ∼5.8 × 10-4 M) indicates that there is no signal due to the hydrogen bonding.10 Thus, it is unambiguous that hydrogen bonding between NsH and CdO units in the two urethane moieties plays an important role in the gel formation process. To investigate the photoresponsiveness of these gels, we performed the photoirradiation of gels using a superhigh-pressure mercury lamp and irradiating through appropriate filters. Cyclohexane gels of compounds 2 and 3 were each prepared in a 1-mm-length cell. Upon irradiation of the cyclohexane gel of 2 with 365-nm light, trans-to-cis isomerization of the azobenzene units occurred. This process collapsed the gel and generated the cis isomer, which precipitated because of its poor solubility in cyclohexane; the precipitated cis isomer could not be isomerized back to the trans form when irradiated at 436 nm.10 In contrast, irradiation of the cyclohexane gel of 3 at 365 nm led to a clear gel-to-sol phase transition as a result of trans-to-cis isomerization and the reverse solto-gel phase transition occurred with cis-to-trans isomerization under irradiation at 436 nm. The corresponding visual images are presented in Figure 3. These phenomena also could be monitored by UV-vis spectroscopy (Figure 4). The change in shape of the absorption spectrum of the (8) The xerogel of 3 was obtained by freezing the cyclohexane gel of 3 and drying it under reduced pressure for 12 h. (9) For estimation of the gel-sol phase transition temperature (Tgel), ∼0.1 mL of a hot solution was put in a screw-capped quartz cell with a 1-mm length. The cell was left at room temperature until gelation completed and was then immersed horizontally in a water bath. The temperature was raised at 1 °C min-1. Tgel was defined as the temperature at which the gel disappeared. (10) See the Supporting Information.
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Figure 2. FTIR spectra of a solution of trans-3 in CCl4 (6.0 mM, dashed line) and of a cyclohexane gel (1.0 wt %, solid line).
Figure 3. Photographic images of (a) the cyclohexane gel of trans-3 (1.0 wt %), (b) the system after irradiation with 365-nm light, and (c) the system from part b after irradiation with 436-nm light.
Figure 4. Changes in the absorptions over time in the UV-vis spectra of the cyclohexane gel of trans-3 (1.0 wt %). The irradiation and the recording of the UV-vis spectra were performed in a 0.01-cm cell. (a) Irradiation of the gel of trans-3 at 365 nm; (b) irradiation at 436 nm of the sample obtained after irradiation at 365 nm for 120 min.
cyclohexane gel of 3 is proof that trans-to-cis isomerization occurs from the gel to the sol state upon irradiation at 365 nm and that the reverse cis-to-trans isomerization from the sol to the gel state occurs when irradiating at 436 nm. To obtain information regarding changes in the hydrogen bonding interactions in these processes, we also investigated the cyclohexane gel of 3 by FTIR spectroscopy before and after irradiation; the results are presented in Figure 5. The FTIR spectrum after irradiation of 365 nm indicates that the intensities of the signals at 3308 and 1686 cm-1, which we assign to NsH and CdO stretching vibration, decreased, but the intensities of those at 3460 and 1728 cm-1 (free NsH and CdO stretching vibrations) increased. After irradiation of this sample at 436 nm, the signals at 3308 and 1686 cm-1 reappeared upon reformation of the gel. It is remarkable that completely reversible light-controlled gel-sol-gel phase transitions can be accomplished using gelator 3 in cyclohexane by breaking and reforming relatively strong hydrogen bonding interactions.
Although it is not yet clear why the central azobenzene unit readily changes its structure in the packing field upon irradiation, a mechanism can be speculated upon from the presumed aggregation pattern and comparing the UV spectrum of the gel with that obtained in solution (Figure 6). In previous reports,6c,11 trans-cis isomerization of azobenzene was not observed in the gel state or in the bilayer membranes, in which the azobenzene units were aligned in the form of H-type aggregates. In those cases, the electronic spectrum of the gel shifted to the blue region. In Figure 6, when comparing the UV spectrum of the cyclohexane gel of trans-3 with that of its solution in cyclohexane, we find that the absorption maximum at 322 nm, which we assign to a π-π* transition of azobenzene, shifted slightly to a longer wavelength (by ∼5 nm) and that the absorption band at ∼375 nm was more intense than that in solution. This result indicates that the azobenzene units are arranged in a different aggregation (11) Shimomura, M.; Kunitake, T. J. Am. Chem. Soc. 1987, 109, 5175.
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Figure 5. Changes in the absorptions in the FTIR spectra of the cyclohexane gel of trans-3 (1.0 wt %). The irradiation experiment and the measurement of the FTIR spectra were performed in an airtight cell using a 1.5-mm spacer. The cyclohexane gel of trans-3 (solid line); the sample obtained after irradiation at 365 nm for 4 h (dotted line); the sample obtained after subsequent irradiation at 436 nm for 1 h (dashed line). (a) NsH stretching vibration region; (b) CdO stretching vibration region.
Figure 6. CD and UV spectra of the cyclohexane gel of trans-3 (1.0 wt %, solid line), of its solution in cyclohexane (5.1 × 10-5 M, dotted line), and of the sample obtained after irradiation of the gel of trans-3 at 365 nm (dashed line).
form from H-type aggregates. One of the possible mechanisms in the present case could be that the aggregates of trans-3 in the gel include space for the central azobenzene unit to change its shape. X-ray diffraction data that will help us to confirm this proposed mechanism are currently under investigation. In agreement with Shinkai’s findings for a related system,6a the asymmetrical structure of our photoresponsive gels could be observed because of the two cholesterol units that behave as chiral auxiliaries. Figure 6 displays
CD and UV spectra of the cyclohexane gel of trans-3, the spectra of trans-3 in cyclohexane solution, and those of the gel after 365-nm irradiation. In the solution, we measured no CD signal, but we observed Cotton effects in the gel state at 239, 305, and 379 nm that correspond to the UV absorption bands arising from the π-π* transition of azobenzene. Because the fibrous network was destroyed upon irradiation at 365 nm, which led to trans-to-cis isomerization, the CD spectrum turned silent. This result indicates that the gelator trans-3 forms aggregates that have chirality caused by the presence of two cholesterol moieties, such that the central azobenzene chromophore is regularly aligned in an asymmetric manner in the gel phase. In conclusion, the azobenzene gelators 2 (m ) 3, n ) 2) and 3 (m ) 4, n ) 2), which are substituted at their meta positions by two urethane units that are linked by methylene chains to two cholesteryl ester moieties, gelatinize cyclohexane, as well as ethanol in the case of compound 3, at concentrations of 1.0 wt %. These cyclohexane gels were active upon irradiation with 365nm light that led to trans-to-cis isomerization of the central azobenzene unit. An FTIR spectroscopic investigation of the cyclohexane gel of compound 3 indicated that these gels are formed by hydrogen bonding between the NsH and CdO groups of the urethane units, which means that ON and OFF modes of switching can be controlled reversibly by irradiation with light through reversible solgel phase transitions. Supporting Information Available: Detailed synthetic methods for the preparation of the three terminal acetylene derivatives and the meta-substituted azobenzenes 1-3, UV-vis spectra of compounds 1-3, images of a cyclohexane gel of compound trans-2, changes in the absorptions in the UV-vis spectrum of trans-2, the FTIR spectrum of the cyclohexane gel of trans-2, and the FTIR spectrum of the solution of trans-3 in cyclohexane. This material is available free of charge via the Internet at http://pubs.acs.org. LA048334F
